And if we sign both him and Koulibaly, we could easily sell 3 out of Rojo, Jones, Bailly, and Smalling, which would recuperate a lot of money both in fees and salaries.
It is hardly that we need many more players if we sign those two. A right back, a right-wing attacker (which could be financed from Sanchez/Lukaku sale) and maybe a midfielder if we decide that Fred is shit for good.
Koulibaly and him, with De Gea behind them would make our defense very strong. I would be happy with either of them anyway, and while Koulibaly is the better player, I would prefer de Ligt.